Method and apparatus for removing liquid from a gas well

ABSTRACT

A method of removing liquid from a gas well having a production tubing with a restriction therein run into the upper portion thereof and wherein a plunger lift system in arranged within the production tubing, the plunger lift system comprising an upper limiting means, a lower limiting means, a plunger that is capable of moving between the upper and lower limiting means, and a one-way valve situated at or above the upper limiting means that is capable of allowing fluid passage therethrough in an upward direction.

The present invention relates to a method and system for removing liquidfrom gas wells.

Most gas wells during their lifetime produce liquids. Liquids may occurin gas wells as a result of condensation, which takes place when thereservoir pressure is insufficient for particular components to remainin the gaseous, state; such components are known as “condensables” andinclude water, propane, butane, pentane, propylene, ethylene andacetylene. A further source of liquids in gas wells may be waterencroachment into the gas producing formation from aquifers or otherwater-bearing zones.

Gas wells which produce liquids are susceptible to liquid loading,whereby a liquid column accumulates at the bottom of the well. Liquidloading takes place when the flow rate of the produced gas isinsufficient to naturally carry liquids to the surface. Liquids arenaturally carried to the surface both as entrained droplets and as aliquid film on the inner surface of the production tubing. However, asthe pressure of gas in the reservoir decreases over time gas flow ratealso decreases. If the flow rate drops below the minimum flow rate atwhich liquids can be lifted out of a particular well by the flowing gasstream—the critical velocity—fall back of liquid occurs and liquidloading begins. Although gas may, for some time, pass through a liquidcolumn as bubbles or slugs, eventually a hydrostatic equilibrium isreached between the reservoir pressure and the back pressure exerted bythe accumulated liquid column. At this point the well is “dead” i.e. itno longer produces gas, although there are still associated reserves. Itis therefore necessary to remove the liquid in order to maintain gasproduction.

Gas wells which experience liquid loading may be low pressure wells,which may sustain liquid loading as soon as production commences.Alternatively, gas wells which undergo liquid loading may be depletedwells, in which the gas flow rate had initially been sufficient to carryliquids to the surface naturally and where production of gas from thegas well has resulted in the reservoir pressure decreasing to such anextent mat liquid loading has commenced.

A widely employed means of removing liquid from gas wells is plungerlift Plunger lift is a cyclic operation wherein a plunger (i.e. apiston) moves reciprocally between two longitudinal positions within awell-production tubing, typically between the lower end of the wellproduction-tubing and a position immediately below, at or immediatelyabove the wellhead. Generally, the plunger travels from the lower end ofthe production tubing towards the wellhead under the propulsion of gaswhich has built up in the well and below the plunger. Gas may build upin the well naturally, or may be injected into the well from thesurface. As the plunger travels towards the wellhead it lifts a liquidcolumn which has accumulated thereabove. This liquid is men removed atthe wellhead, for example via outlet pipes. Typically, the well isshut-in (i.e. production is stopped) thereby providing an enclosedsystem and the plunger is allowed to fall through the production tubingunder the influence of gravity. When the plunger reaches a column ofliquid at the lower end of the production tubing the liquid bypasses theplunger via an annulus between the plunger and the inner-surface of theproduction tubing and/or through a longitudinal channel through theplunger and accumulates above the plunger. The well is then put backinto production, and the pressure that has built up below the plungerlifts the plunger and the liquid column towards the wellhead. Theannulus between the plunger and the inner surface of the tubing is suchmat although liquid bypass is tolerated when the plunger falls throughthe liquid column, on lift of the plunger mere is sufficient gasturbulence in the annulus to create a liquid seal which prevents, atleast to some extent, fallback of liquid from above the plunger.

Various types of plunger may be employed in a plunger lift system.Two-piece plungers, for example, comprise two members which, whencoupled together, are capable of lifting liquids. The two members of theplunger are capable of independently falling through an upwardly movingstream of gas or a multiphase stream of gas and liquid withoutsubstantially impeding the upward flow of the stream. Thus, a two-pieceplunger can be dropped into a gas well whilst the well is in production,thereby reducing the period for which the well must be shut in, oreliminating the need for shutting in the well. This may significantlyspeed up plunger lift cycles.

Various other means of removing liquids from gas wells are available;however, all have associated disadvantages.

For example, reducing the size of the production tubing, therebyreducing the cross-sectional flow area, can increase the gas flow ratein a given gas well. However, replacing tubing is an expensive procedureand further tubing reduction may be required as the pressure of gasdeclines over time.

Various down-hole pumping devices are also available to remove liquidsfrom gas wells; however, this requires energy input and expensiveequipment for operation. Further, pumps can be readily damaged bysolids, such as sand, which may be present in the gas well. However,such down-hole pumping devices experience problems with free gas whichinterferes with pumping the liquids.

Alternatively, a surfactant or mixture of surfactants may be injectedinto a gas well to form a foam, which reduces the surface tension of theliquid such that the foam may be lifted at a lower critical velocitythan is required for lifting the “un-foamed” liquid. However, althoughsurfactants are useful in de-watering gas wells, liquid hydrocarbons donot foam well and the produced liquid may require continual agitation tomaintain foaming. A disadvantage of injecting a surfactants or mixtureof surfactants into a gas well is the cost of the chemicals andpotential technical problems associated with the deployment of thesechemicals. Also, the installation of a permanent injection facility canbe costly and technically challenging. Furthermore, batch treating(dumping surfactant into a well) adds to the hydrostatic liquid columnresulting in a limited “time-window” in which the treatment iseffective.

Plunger lift systems are advantageous because they can operate withoutthe input of energy and their installation is relatively cheap. Inaddition, there are methods available to determine whether plunger liftwill work in a particular well prior to installation. Also, if, for somereason, a plunger lift system fails to work, it can be relatively easilyremoved from the well by, for example, wireline retrieval methods.

U.S. Pat. No. 2,676,547 and U.S. Pat. No. 2,160,291 each describe anapparatus for raising oil or other fluid from wells. The apparatuseshave two plungers, each of which carries its load up only part of thewell. U.S. Pat. No. 7,080,692 discloses a plunger lift tool which ispositioned in a well between a lower and an upper plunger.

Some existing plunger lift systems do, however, suffer from at least onesignificant disadvantage: since the plunger must be able to move betweenthe lower and upper limiting means, the production tubing that is runinto the well must hot comprise any restrictions which would preventthis. Thus, some existing plunger lift systems may not be employed inany gas well where the production tubing has such a restriction. Suchrestrictions may include down-hole valves, or narrowing of theproduction tubing. In particular, existing plunger lift systems may notbe employed in gas wells where a sub-surface safety valve is present inthe production tubing.

There remains a need for an apparatus and method which overcome or atleast mitigate these problems. In particular, mere remains a need for animproved apparatus and method for removing liquid from gas wells whichhave a restriction such as a sub-surface safety valve within theproduction tubing.

According to a first aspect of the present invention there is provided amethod of removing liquid from a gas well having a production tubingwith a restriction therein run into the upper portion thereof andwherein a plunger lift system is arranged within the production tubing,the plunger lift system comprising an upper limiting means, a lowerlimiting means, a plunger that is capable of moving between the upperand lower limiting means, and a one-way valve situated at or above theupper limiting means that is capable of allowing fluid passagetherethrough in an upward direction but not in a downward direction; themethod comprising the steps of:

-   -   a) allowing liquid to accumulate in the gas well with the        plunger held at the upper limiting means until a column of        liquid forms in the production tubing above the lower limiting        means;    -   b) releasing the plunger such that it falls towards the lower        limiting means through the column of liquid until it engages        with the lower limiting means;    -   c) allowing gas pressure to build up in the gas well below the        plunger until the plunger and column of liquid are forced        towards the upper limiting means such that the plunger engages        with the upper limiting means and at least a portion of the        column of liquid passes through the one-way valve; and    -   d) repeating steps a) to c).

It is envisaged that gas pressure may also build up, in step c), in theannulus between the production tubing and the casing of a cased gaswell.

The method of the present invention may be employed in a deviated wellprovided that the angle of deviation of the well from the vertical isless than 60°, preferably less than 30°, such that the plunger iscapable of falling under gravity through the production tubing.

The upper and lower limiting means are simply means which prevent theplunger, travelling further than required. However, it is preferred matthe upper and lower limiting means do not impede fluid flow in theproduction tubing.

The one way-valve may be situated either above or below the restrictionin the production tubing, preferably immediately below the restriction.It is also envisaged that the one-way valve may be situated at the upperlimiting means, and may in fact act as the upper limiting means. Wherethe restriction in the production tubing is a downhole valve, such as asub-surface safety valve of an off-shore gas well, the one-way valve maybe separate from the downhole valve, for example it may be arrangedabove the one-way valve, or it may be incorporated into the downholevalve. Accordingly, the downhole valve may be the restriction in theproduction tubing and also act as the upper limiting means.Alternatively or additionally, the downhole valve may act as the one-wayvalve. This provides for relatively easy access if maintenance isrequired.

Where the one-way valve lies above the upper limiting means, it ispreferred that the volume of the column of liquid that forms in theproduction tubing above the lower limiting means is substantiallygreater than the volume of the production tubing between the upperlimiting means and the one way-valve such that the plunger forces asubstantial portion of the column of liquid through the one-way valve.It is therefore preferred that the one-way valve lies immediately abovethe upper limiting means.

Preferably, the plunger forces at least 75% of the column of liquidthrough the one-way valve, preferably, at least 95%.

Preferably, the lower limiting means is located within the productiontubing at or near the lower end thereof.

During step (a), the gas well is in production. However, a gas stream ora multiphase stream of gas and liquid is capable of by-passing theplunger that is retained at the upper limiting means via an annulusbetween the plunger and the inner surface of the production tubing. Thegas stream or multiphase stream then passes through the one-way valveand to the wellhead of the gas well.

Preferably, the gas well is shut-in during step (b). Preferably, die gaswell is opened-up during step (c).

The one-way valve of the plunger lift system prevents the liquid thathas been forced therethrough from falling back down the productiontubing. As would be well known to the person skilled in the art ofdeliquification, the volume of gas increases with decreasing depth dueto expansion of the gas at lower pressures. Accordingly, the velocity ofthe gas that has passed through the one way valve may be higher than thevelocity of gas in the lower end of the production tubing. This can leadto the phenomenon that, in upper parts of a gas well, the velocity ofthe gas may exceed the critical velocity. Where the velocity of the gasstream or of the multiphase stream that passes through the one-way valveis above the critical velocity, liquid will be naturally lifted fromabove the one-way valve to the wellhead.

Where the flow rate of the gas stream or of the multiphase stream thatpasses through the one-way valve is below the critical velocity, liquidwill accumulate above the one-way valve. Initially, this liquid may besufficiently agitated to allow for the passage of gas. However,artificial lift may eventually be required to remove the liquid thataccumulates in the production tubing above the one-way valve. Owing tothe potential increased velocity of the gas stream or the multiphasestream above the one way valve and/or the decreased distance to lift theliquid, the chosen method of artificial lift that is employed is likelyto be more effective and/or economical when used in combination with themethod of the present invention than if that artificial lift method wasused alone to lift liquid from the lower end of the production tubing.

It is therefore envisaged that the method of the present invention mayfurther comprise the step of artificially lifting liquid thataccumulates in the production tubing above the one-way valve to thewellhead of the gas well. For example, a surfactant or a mixture ofsurfactants may be added to any liquid which accumulates above theone-way valve thereby forming a foam which is lifted to the wellhead bythe flow of the gas stream or multiphase stream that passes through theone way valve.

Alternatively, a further plunger lift system may be employed to lift anyliquid that accumulates in the production tubing above the one wayvalve. Accordingly, the method of the present invention may furthercomprise the following steps:

-   -   e) arranging a further plunger lift system within the production        tubing above the restriction in the production tubing and above        the one-way valve, the further plunger lift system comprising an        upper limiting means situated at, immediately above or        immediately below the wellhead, a lower limiting means situated        at or above the restriction within the production tubing and at        or above the one-way valve, and a plunger that is capable of        moving between the upper and lower limiting means;    -   f) allowing a further column of liquid to accumulate above the        restriction within the production tubing and above the one        way-valve with the plunger of the further plunger lift system        held at the upper limiting means;    -   g) releasing the plunger of the further plunger lift system such        that the plunger falls towards the lower limiting means of the        further plunger lift system and through the further column of        liquid until it engages with the lower limiting means of the        further plunger lift system;    -   h) allowing gas pressure to build up below the plunger of the        further plunger lift system until the plunger of the further        plunger lift system and the further column of liquid are forced        towards the upper limiting means of the further plunger lift        system such that the plunger engages with the upper limiting        means and removing at least a portion of the column of liquid at        the wellhead; and    -   i) repeating steps e) to h).        Preferably the gas well is shut-in during step (g). As would be        evident to the person skilled in the art, the gas well is        opened-up during step (h).

According to a second aspect of the present invention, there is providedan apparatus for removing liquids from a gas well having a productiontubing with a restriction therein run into the upper portion thereof,the apparatus comprising a plunger lift system arranged within theproduction tubing wherein the plunger lift system comprises: an upperlimiting means, a lower limiting means, a plunger that is capable ofmoving between the upper and lower limiting means, and a one-way valvethat is capable of allowing fluid passage therethrough in an upwarddirection but not in a downward direction wherein the one-way valve issituated at or above the upper, limiting means, the upper limiting meansis situated at or below the restriction in the production tubing, andthe lower limiting means is situated at a location where, duringproduction of gas from the gas well, a column of liquid accumulates inthe production tubing.

The plunger lift system may further comprise a retaining means forholding the plunger at or near the upper limiting means until it isrequired to fall. For example, such retaining means may be amechanically or electrically actuated retaining-means, controllable fromthe surface. Alternatively, the upper limiting means may comprise anelectromagnetic retaining means, also controllable from the surface.Electricity may be supplied to such retaining means by batteries, or byan electrical cable run, for example, through an annulus between theproduction tubing and the well-casing and men through the wall of theproduction tubing.

The plunger lift system may further comprise a means for detecting whenthe volume of the column of liquid (liquid loading) is sufficiently highthat the plunger is required to fall towards the lower limiting meansand through the column of liquid until it engages with the lowerlimiting means. Such means may, for example, be a pressure measurementdevice, located at the lower limiting means, wherein the pressuremeasurement is correlated with liquid loading. Thus, the higher thepressure difference between the wellhead pressure and the downholepressure at the lower limiting means, the more liquid is present in theproduction tubing.

The plunger lift system may further comprise means for detecting whenthe plunger has reached the lower limiting means. Such means include,for example, electro-magnetic detectors, impact detectors, and acousticdetectors.

The upper limiting means of the plunger lift system is preferably therestriction in the production tubing. Alternatively, the upper limitingmeans may be a separate restriction within the production tubing.

As discussed above, the one-way valve may be situated above or below therestriction in the production tubing. Preferably the one-way valve issituated below the restriction. More preferably the one-way valve actsas the upper limiting means. Also, as discussed above, the one-way valvemay act as the restriction in the production tubing. The restriction maybe a sub-surface safety valve, and this can be separate from the one-wayvalve or it can act as the one-way valve and/or as the upper limitingmeans.

In order to prevent damage to the plunger and/or the production tubing,the plunger lift system may further comprise shock absorber means forabsorbing shock when the plunger impacts upon the lower and upperlimiting means. The shock absorber means may act as the upper and/orlower limiting means. For example, the upper and/or lower limiting meansmay be a resilient bumper, such as a bumper spring. Alternatively one orboth of the shock absorbers may be attached, either directly orindirectly, to the plunger itself, for example a bumper spring may beattached to the upper and/or lower surfaces of the plunger.

Typically, the plunger of the plunger lift system is cylindrical inshape where the longitudinal axis of the cylinder is aligned with thelongitudinal axis of the production tubing. Suitably, the plunger iscomposed of a dense material, such as galvanised titanium or steel.Frequently the cylindrical surface of the plunger is provided with ribsor grooves in order to aid turbulence in the annulus between the plungerand the production tubing. Typically, the cylindrical surface of theplunger is provided with circumferential or helical ribs or grooves.

Preferably the plunger of the plunger lift system of the presentinvention is a two-part plunger. A first part of the plunger may have achannel extending there-through and a second part of the plunger may beadapted to close off said channel when the first and second parts arebrought into contact. The first part can be held at the upper limitingmeans in step a) (and step f if present), for example by the retainingmeans, whereas the second part is separable from the first part and canfall towards the lower limiting means during step a) (and step f ifpresent).

Advantageously, the first part can comprise a cylindrical member withwhich the second part can sealingly engage thereby sealing the channel,such that passage of fluids through the channel is either prevented oris substantially reduced.

The channel may be arranged centrally in the cylindrical first member ormay be offset. It is preferred that the longitudinal axis of the channelis aligned with the longitudinal axis of the cylindrical first member.In use, the two-piece plunger is arranged in the production tubing withthe longitudinal axis of the cylindrical first member aligned with thelongitudinal axis of the production tubing. Generally, the cylindricalfirst member is arranged above the second part in the production tubing.Preferably, the lower surface of the cylindrical first member is shapedto complement the shape of the second part, for example, the second partmay be spherical and the lower surface of the cylindrical first membermay be concave—this ensures that sealing engagement of the first andsecond parts can be readily achieved. Alternatively, the second part maybe a rod which fits inside the channel of the cylindrical first member.Such rods may be provided with stabilisers, for example a series ofoutwardly extending arms at the base of the rod, such that the rodremains aligned with the channel of the cylindrical first member.

In use, the cylindrical first member of the two-piece plunger is held byretaining means at the upper limiting means while the second memberfalls down the production tubing toward the lower limiting means. As thesecond member falls, a gas stream or a multiphase stream of gas andliquid is able to flow upwardly around it. Thus, the second member maybe allowed to fall through the production tubing while the gas wellremains in production. When required, the cylindrical first member maybe released from the retaining means. As the cylindrical first memberfalls down the production tubing it allows upward flow of a gas streamor a multiphase stream of gas and liquid through the channel.Accordingly, the cylindrical first member may also be released from theretaining means while the gas well remains in production. When thecylindrical first member reaches the column of liquid that hasaccumulated above the lower limiting means, liquid will be forcedthrough the channel of the cylindrical first-member in addition toby-passing the plunger via an annulus between the cylindrical firstmember and the inner surface of the production tubing. When thecylindrical first member reaches the lower limiting means it sealinglyengages with the second member, substantially preventing any flow offluids through the channel of the cylindrical first member. The firstand second members will then remain coupled at the lower limiting meansuntil sufficient gas pressure has built up below the two-piece plungerto lift both the two-piece plunger and the liquid column above thetwo-piece plunger up the production tubing towards the upper limitingmeans such that at least a portion of the column of liquid passesthrough the one-way valve. There may be sufficient gas pressure forlifting the two-piece plunger and associated liquid column as soon asthe first member reaches the lower limiting means. Alternatively, thewell may be shut-in until sufficient gas pressure has built up.

Typically the upper limiting means is provided with a decoupling meansfor decoupling the sealingly-engaged first and second members of thetwo-piece plunger thereby unsealing the channel of the cylindrical firstmember. It is preferred that the decoupling means does not preventupward passage of fluids through the channel of the cylindrical firstmember of the two-piece plunger. Typically, the decoupling meansprotrudes downwardly from the upper limiting means and is capable ofbeing inserted through the channel of the first member to decouple thetwo-piece plunger by forcing the second member away from sealingengagement with the channel of the cylindrical first member. Typically,the downwardly protruding decoupling means is a pipe having a lengthgreater than the length of the channel in the cylindrical first member.Alternatively, the downwardly protruding decoupling means may be a rod,which is of narrower diameter than the channel and has a length that isgreater than the length of the channel of the cylindrical first member.Initially, the gas flow rate may be too high for the second member ofthe two piece plunger to fell through the production tubing. Thus, thesecond member may be held by gas flow adjacent the channel through thecylindrical first member. However, the second member does not block thechannel, as the rod has forced the second member away from sealingengagement with the channel such that fluid can flow around the secondmember and through an annulus between the wall of the channel and therod. Where the downwardly protruding decoupling means is a pipe, it ispreferred that the lower end of the pipe is shaped so as to mitigate therisk of the second member blocking the channel in the cylindrical firstmember to flow of fluid therethrough, for example, the lower end of thepipe may be cut at an angle, or there may be perforations in the lowerend of the pipe. As gas flow declines, owing to the accumulation of thecolumn of liquid in the production tubing, the second member will falltoward the lower limiting means of the plunger lift system.

Suitably the apparatus of the present invention may comprise a furtherplunger lift system comprising an upper limiting means situated at ornear the wellhead and a lower limiting means situated at or above therestriction within the production tubing and at or above the one-wayvalve. Where the one-way valve is situated below the restriction in theproduction tubing, the lower limiting means of the further plunger liftsystem is suitably the restriction. Where the one-way valve is situatedabove the restriction in the production tubing, the lower limiting meansof the further plunger lift system is suitably the one-way valve.Preferably the further plunger lift system additionally comprises aretaining means for holding the plunger at or near the upper limitingmeans. Preferably the further plunger lift system also includes a meansof detecting when sufficient liquid has accumulated above the one-wayvalve such that the plunger is required to fall. A means of detectingwhen the plunger of the further plunger lift system has reached thelower limiting means is preferably provided. The further plunger liftsystem may also comprise shock absorbing means, as described above.Preferably the plunger of the further plunger lift system is a two-pieceplunger of the type described above in which case, the upper limitingmeans of the further plunger lift system is provided with a decouplingmeans.

Where a gas well comprises more than one restriction within theproduction tubing, a corresponding number of plunger lift systemsaccording to the invention may be employed, for successively liftingliquid past each of the restrictions within the production tubing and tothe wellhead.

Where the apparatus of the present invention comprises at least onefurther plunger lift system, it is envisaged mat the apparatus mayfurther comprise a timing device. Such a timing device may allow theplungers of the systems to be released at different times. In this way,a lower plunger can transfer liquid through a restriction beforeanother, upper, plunger starts it ascent. Thus ‘handover’ of liquid fromone plunger to the next is efficient and unnecessary lifting of one ofthe plungers, in particular when a plunger has little, or no, liquidabove it, can be prevented.

The invention will now be described by way of example only and withreference to FIG. 1 which represents, in simplified cross-section, aproduction tubing of an off-shore gas-well having an apparatus accordingto the invention arranged therein.

Referring to FIG. 1, the apparatus comprises: a first and a secondplunger lift system arranged within a production tubing (4) below andabove a sub-surface safety valve (5) respectively. The first plungerlift system comprises a plunger (1), an upper limiting means (2), alower limiting means (3), and a one-way valve (6) situated above theupper limiting means (2) and below the sub-surface safety valve (5). Inan alternative, the sub-surface safety valve is the one-way valve andalso the upper limiting means. The second plunger lift system comprisesa plunger (7), an upper limiting means (not shown) and a lower limitingmeans (8), situated above the sub-surface safety valve (5). The plunger(1) of the first plunger lift system is a two-piece plunger comprising acylindrical first member (1) and a spherical second member (9) whereinthe cylindrical first member (1) has a channel (10) extendinglongitudinally therethrough and a concave lower surface (not shown). Theupper limiting means (2) of the first plunger lift system is providedwith a decoupling means (11) for releasing the spherical second member(9) of the two-piece plunger from sealing engagement with the concavelower surface of the cylindrical first member. The decoupling means (11)is a pipe that extends downwardly from the upper limiting means (2) andhas its lower end cut at an angle. A retaining means (notshown)—preferably a mechanical tool that is triggered by pressure—isprovided for holding the first member of the two-piece plunger (1) at ornear die upper limiting means (2). The lower limiting means (3) of thefirst plunger lift system is optionally provided with a shock absorbingmeans. The plunger (7) of the second plunger lift system is also atwo-piece plunger (7), comprising a cylindrical first member (7) and aspherical second member (12) wherein the cylindrical first member (7)has a channel (13) extending longitudinally therethrough and a concavelower surface (not shown). The upper limiting means (not shown) of thesecond plunger lift system is the wellhead or may be located near thewellhead and the upper limiting means is provided with a decouplingmeans (not shown), for example, a rod protruding downwardly from thewellhead; and a retaining means (not shown) for holding the first memberof the two-piece plunger of the second plunger lift system at or nearthe upper limiting means (not shown) of the second plunger lift system.Preferably, the retaining means is a conventional lubricator typearrangement. The lower limiting means (8) of the second plunger liftsystem is optionally provided with a shock-absorbing means.

In use, a column of liquid (14) is allowed to accumulate above the lowerlimiting means (3) of the first plunger lift system while thecylindrical first member (1) of the two-piece plunger is held at dieupper limiting means (2) by the retaining means (not shown). When thevolume of the column of liquid (14) reaches a critical volume at whichgas production declines to an unacceptable level, the gas well isgenerally shut-in and the first member (1) of the two-piece plunger isreleased from the retaining means (not shown) and is allowed to fall. Asthe first member (1) falls, gas or a multiphase mixture of gas andliquid passes through the channel (10). When the cylindrical firstmember (1) reaches the column of liquid (14), liquid is forced throughthe channel (10) and through an annulus between the plunger (1) and theinner surface of the production tubing (4) until the first member (1)sealingly engages with the second cylindrical member (9) of thetwo-piece plunger. Gas pressure is then allowed to build up below thetwo-piece plunger (1 & 9) and, if applicable, in the annulus between theproduction tubing and casing of a cased gas well, until the two-pieceplunger (1 & 9) and the column of liquid (14) are forced towards theupper limiting means (2). Generally, the two-piece plunger is forcedtowards the upper limiting means (2) when the well is opened (offset bythe initial pressure drop when opening up the well). As the two-pieceplunger (1 & 9) approaches the upper limiting means (2) at least aportion of the liquid column above the plunger will pass through theone-way valve. When the two-piece plunger (1 & 9) of the first plungerlift system reaches the upper limiting means (2), the second member ofthe two-piece plunger (9) will be forced away from the first member (1)by the decoupling means (11) and the retaining means for holding theplunger at the upper limiting means (not shown) is actuated. If the gasflow rate is initially too high for the second member (9) of thetwo-piece plunger to fall, the angled end of the downwardly protrudingpips (11) will prevent the second member (9) of the two-piece plungerblocking the pips (11) to passage of gas or multiphase fluid. As gasflow rate declines (for example due to further liquid loading) thesecond member (9) of the two-piece plunger will fall to the lowerlimiting means (3) of the first plunger lift system. The cycle may thenbe repeated.

Liquid which is lifted through the one-way valve (6) by the firstplunger lift system will accumulate above the one-way valve as a liquidcolumn (15). Once the volume of the liquid column (15) above the one-wayvalve (6) exceeds a certain critical volume at which gas productiondeclines to an unacceptable level, the first member (7) of the two-pieceplunger of the second plunger lift system is released from the retainingmeans (not shown) and allowed to fall. When the first member (7) of theplunger of the second plunger lift system reaches the column of liquid(15), liquid is forced through the channel 13) extending longitudinallythrough the first member (7) and through an annulus between the plunger(7) and the inner surface of the production tubing (4) until the firstmember (7) sealingly engages with the second member (12) of thetwo-piece plunger of the second plunger lift system. Gas is then allowedto build up below the plunger (7 & 12) of the second plunger lift systemuntil the plunger and the column of liquid (15) are forced towards theupper limiting means (not shown) of the second plunger lift system. Whenthe plunger of the second plunger lift system arrives at the wellhead,liquid is removed at the well head (not shown) by an outlet pips. On theplunger's arrival at the well head, the retaining means for holding thefirst member (7) at the well head (not shown) can bs actuated and theprotruding rod forces the spherical second member (12) of the two-pieceplunger away from sealing engagement with the channel of the cylindricalfirst member, such that the second member (12) may fell to the lowerlimiting means (8). The cycle may then bs repeated.

Optionally the retaining means may be connected to a timer device (notshown) such that the first members (1 & 7) of the first and secondplunger lift systems may be allowed to fall at different times.

1-9. (canceled)
 10. A method of removing liquid from a gas well having aproduction tubing with a restriction therein run into the upper portionthereof and wherein a plunger lift system is arranged within theproduction tubing, the plunger lift system comprising an upper limitingmeans, a lower limiting means, a plunger that is capable of movingbetween the upper and lower limiting means, and a one-way valve situatedat or above the upper limiting means that is capable of allowing fluidpassage therethrough in an upward direction but not in a downwarddirection; the method comprising the steps of: a) allowing liquid toaccumulate in the gas well with the plunger held at the upper limitingmeans until a column of liquid forms in the production tubing above thelower limiting means; b) releasing the plunger such that it fallstowards the lower limiting means through the column of liquid until itengages with the lower limiting means; c) allowing gas pressure to buildup in the gas well below the plunger until the plunger and column ofliquid are forced towards the upper limiting means such that the plungerengages with the upper limiting means and at least a portion of thecolumn of liquid passes through the one-way valve; and d) repeatingsteps a) to c).
 11. A method as claimed in claim 10, wherein the plungercomprises two parts, a first part which has a channel extendingthere-through and which is held at the upper limiting means in step a),and a second part which is separable from the first part so as to falltowards the lower limiting means during step a), the second part beingadapted to close off the channel of the first part when the first andsecond parts are brought into contact.
 12. A method as claimed in claim10, wherein the gas well has a casing and in step c) gas pressure buildsup in the annulus formed between the casing and the production tubing.13. A method as claimed in any claim 10 which further comprises thesteps of: e) arranging a further plunger lift system within theproduction tubing above the restriction in the production tubing andabove the one-way valve, the further plunger lift system comprising anupper limiting means situated at, immediately above or immediately belowthe wellhead, a lower limiting means situated at or above therestriction within the production tubing and at or above the one-wayvalve, and a plunger that is capable of moving between the upper andlower limiting means; f) allowing a further column of liquid toaccumulate above the restriction within the production tubing and abovethe one way-valve with the plunger of the further plunger lift systemheld at the upper limiting means; g) releasing the plunger of thefurther plunger lift system such that the plunger falls towards thelower limiting means of the further plunger lift system and through thefurther column of liquid until it engages with the lower limiting meansof the further plunger lift system; h) allowing gas pressure to build upbelow the plunger of the further plunger lift system until the plungerof the further plunger lift system and the further column of liquid areforced towards the upper limiting means of the further plunger liftsystem such that the plunger engages with the upper limiting means andremoving at least a portion of the column of liquid at the wellhead; andi) repeating steps e) to h).
 14. A method according to claim 13,including the further step of timing the release of the plunger in stepg) so that said plunger engages the lower limiting means of said furtherplunger lift system after said portion of the column of liquid passesthrough the one-way valve in step c).
 15. An apparatus for removingliquids from a gas well having a production tubing with a restrictiontherein run into the upper portion thereof, the apparatus comprising aplunger lift system arranged within the production tubing wherein theplunger lift system comprises: an upper limiting means, a lower limitingmeans, a plunger that is capable of moving between the upper and lowerlimiting means, and a one-way valve that is capable of allowing fluidpassage therethrough in an upward direction but not in a downwarddirection, wherein the one-way valve is situated at or above the upperlimiting means, the upper limiting means is situated at or below therestriction in the production tubing, and the lower limiting means issituated at a location where, during production of gas from the gaswell, a column of liquid accumulates in the production tubing, theplunger lift system further comprising retaining means for releasablyholding the plunger at or near the upper limiting means.
 16. Theapparatus of claim 15, wherein the plunger comprises a first part whichhas a channel extending there-through and which is releasably engageablewith the retaining means, and a second part which is separable from thefirst part and which is adapted to close off the channel of the firstpart when the first and second parts are brought into contact.
 17. Theapparatus of claim 15, further comprising a further plunger lift systemarranged within the production tubing above the first plunger liftsystem, the further plunger lift system comprising: an upper limitingmeans, a lower limiting means and a plunger that is capable of movingbetween the upper and lower limiting means, wherein the lower limitingmeans is situated at or above the restriction in the production tubing,the further plunger lift system additionally comprising retaining meansfor releasably holding the plunger at or near the upper limiting means.18. The apparatus of claim 17, further comprising a timer device foractivating the retaining means of the further plunger lift system torelease the plunger of the further plunger lift system.